Precision time interval measurements using an electronic counter allow accurate measurement of the elapsed time between start and stop events. Typically, instruments include two counters, a time counter to accumulate events from a stable clock, and an event counter to accumulate events from an input signal. Dividing the number of events in the event counter by the result in the time counter provides the average frequency of the input signal. The two counters are started and stopped by a signal called a gate. If the gate is synchronized with the clock, the gate time is controlled exactly and the number of input signal events is counted with some uncertainty (plus or minus one event). If the gate is synchronized with the input signal, the number of input signal events is counted exactly and the elapsed time is counted with some uncertainty (plus or minus one clock period). Increased accuracy can be gained by averaging over more than one input signal event, and in the case of repetitive signals, by statistically averaging over several measurements.
For frequency measurement of pulsed signals, for example the RF bursts in a pulsed radar signal, the measurement must be started and stopped while the pulsed signal is present for the frequency measurement to be correct. Otherwise, the time counter is accumulating clock events when the event counter has no corresponding input signal events to accumulate.
Conventional counters assured that the gate signal was opened and closed within the duration of the pulsed input signal by using the envelope of the pulsed input signal to generate the gate signal. This technique involves the use of expensive delay lines, and results in some uncertainty of the timing relationship of the gate to the input pulse.
FIG. 1 shows an example of the wave forms in the conventional technique. Wave form A is the pulse envelope, wave form B is the RF pulse, delayed by 15 nsec using a delay line to preserve the signal. Wave form C is the envelope delayed by 30 nsec. Wave form D is the gate, produced by combining wave forms A and C in a logical AND gate. The result is a gate signal D that opens 15 nsec after the start of the pulse and closes 15 nsec. before the end of the pulse.
In addition to the average frequency of the pulsed signal, it may be desirable to measure the behavior of the input signal frequency over time, the "frequency profile". This kind of measurement is important for transient or swept frequencies, e.g., chirped radar pulses. This measurement is conventionally made by using a narrow gate from an external pulse generator that is "swept through" the input signal pulse in a series of measurements.
A major drawback of this method is that narrowing the gate reduces the accuracy of the frequency measurement, while widening the gate smooths out the possible frequency variations over time, and thus reduces the ability to measure rapid frequency variations in the input signal. A narrow gate reduces the frequency accuracy in two ways: random and systematic errors. In the first case, random truncation error in the time measurement becomes a more significant fraction of the gate, thereby increasing random fractional frequency error. In the second case, systematic errors from start to stop of the gate tend to react with each other more with proximity, making calibration by simple subtraction of fixed errors less accurate.
Another drawback is that because of the delays in generating the external gate pulses, it is difficult to open the gate near the leading edge of a pulse. The gate can be delayed to the next pulse, but jitter in the timing of the delay generator or in the pulse repetition interval lead to uncertainty in the position of the gate with respect to the pulse.